A domestication related mutation in the thyroid stimulating hormone receptor gene (TSHR) modulates photoperiodic response and reproduction in chickens
Introduction
One of the most striking characteristics of domesticated animals is the reduced seasonality in reproduction (Trut et al., 2009). While non-domesticated animals living in temperate regions typically reproduce on a seasonal basis, the ability to have offspring throughout the year is apparent in most domestic species such as sheep, cattle, pigs, cats and dogs (Faya et al., 2011, Setchell, 1992, Trut, 1999). Seasonal reproduction depends on stimuli varying across the year, such as day length, and ensures that offspring is born at a time of year when food is abundant, thereby increasing the chances of survival of the offspring. During domestication, less strict seasonal reproduction is highly beneficial, and has therefore been under selection in most species. Furthermore, the adaptation to a captive environment tends to modify an array of other traits as well, including morphology, physiology, development and behavior (Jensen, 2006, Price, 1999).
The Red Junglefowl (RJF, Gallus gallus) is the ancestor of the domestic chicken (West and Zhou, 1988). The bird lives in Southeast Asia and shows a robust photoperiodic response (Ono et al., 2009). During spring and summer RJF females lay clutches of 3–7 eggs which they incubate (Collias and Collias, 1967). The domestication of the chicken started already 6000 B.C. (West and Zhou, 1988) and has caused obvious phenotypic changes as a consequence of underlying genetic alterations. For example, the White Leghorn (WL, Gallus gallus domesticus) is a domesticated breed selected for high egg production, which lays eggs throughout the year, independent of season, and rarely incubates them.
In a seminal study, RJF as well as eight different populations of domestic chickens were subjected to whole-genome resequencing, in order to identify selective sweeps and candidate mutations of importance for domestication (Rubin et al., 2010). The most striking sweep was detected at the locus for the thyroid stimulating hormone receptor (TSHR), and was shared among all domestic chickens in the study. A missense mutation causing a non-conservative amino acid substitution was suggested as the candidate mutation in the sweep (Rubin et al., 2010). Ancient DNA studies have shown that the mutation was present already in classical Greek chickens which indicates that it is old, but data also suggest that the sweep allele was not fixed in European chickens until much later in time, and took place only 500 years ago due to strong human-mediated selection (Girdland Flink et al., 2014).
The TSHR plays an important role in the hypothalamic-pituitary–gonadal axis, which regulates photoperiodic responses and seasonal reproduction. The photoperiodic signal transduction in birds starts with stimulation of deep brain photoreceptors (Nakane et al., 2010, Nakane and Yoshimura, 2010) and is further translated into neuroendocrine responses transmitted to the pars tuberalis where it induces thyroid stimulating hormone β subunit (TSHB) expression. TSHB associates with the common glycoprotein alpha subunit (CGA), which is rhythmically expressed, to form thyroid stimulating hormone (TSH). TSH from the pars tuberalis then connects to TSHR expressed in the ependymal cells and induces the expression of type 2 deiodinase (DIO2), that converts thyroxine (T4) to bioactive 3, 5,3′-triiodothyronine (T3) (Ikegami and Yoshimura, 2012, Nakane and Yoshimura, 2010, Nakao et al., 2008). DIO2 is expressed during long day conditions, and during short days there is a reciprocal switch, where DIO2 is down regulated and the type 3 deiodinase (DIO3), which inactivates T3, is up-regulated. Long day induced T3 production causes secretion of luteinizing hormone and follicle stimulating hormone and thereby further gonadal growth (Yasuo et al., 2005). The molecular mechanisms of seasonal reproduction in birds have mainly been investigated in Japanese quail, but studies have shown that the Red Junglefowl shows similar responses (Ono et al., 2009).
These facts together indicate that the TSHR mutation may be a true “domestication gene”. In previous studies, we have shown that the mutation delays hatching by about one day, and also modifies a range of behavior in a way typically associated with domestication (Karlsson et al., 2015). In the present study we focus on the effect on photoperiodic responses. To study this we have used birds generated from a Locus Controlled Advanced Intercross Line (LAIL). The chickens were bred from an F10-generation of an advanced intercross line between RJF and WL and were either homozygous for the wild type TSHR allele (w/w) or for the mutant allele (d/d). This allowed us to study the effect of the genotype at the TSHR locus against the random RJF x WL hybrid background genotype achieved by accumulating recombinations during the previous ten generations.
We observed reproductive traits as well as TSHB, TSHR, DIO2 and DIO3 gene expression during altered day length (photoperiod) in females and males with alternative genotypes at the TSHR locus in order to find consistent differences between genotypes. In addition, we included a small number of purebred WL and RJF females kept under natural day length as a proof of principle. Furthermore, the presence of the TSHR mutation was investigated in a number of Swedish landrace chickens, in order to evaluate the presumed age and spread of the mutation. We hypothesized that animals with different TSHR genotypes would differ in their reproductive responses to altered day length, and that this would be mirrored in the expression of genes involved in the photoresponse cascade. Moreover, we predicted that the mutation would be present at a high rate in Swedish landraces, reflecting our hypothesis that the mutation was selected early during domestication.
Section snippets
Ethical note
The study was approved by Linköping local Ethical committee of The Swedish National Board for Laboratory Animals (approval No. Lkp 85–07).
Advanced Intercross: TSHR chickens
The main part of this study was performed on offspring from the F10 generation of an advanced intercross between WL and RJF. The WL-line (SLU 13) has a long history of selection for egg production traits and originated from a Scandinavian selection and crossbreeding experiment (Liljedahl et al., 1979). The Red Junglefowl originated from a Swedish zoo
TSHR males
There was a significant increase in paired testicular weight from sampling session one to two for the TSHR w/w genotype (U = 0.0, n1 = 5, n2 = 4, P = 0.02), but no differences between genotypes within each sampling occasion (Fig. 1a). As shown in Fig 1b, there was a significant decrease in TSHB mRNA levels from sampling occasion one to two (U = 0.0, n1 = 5, n2 = 4, P = 0.02) and from two to three (U = 0.0, n2 = 4, n3 = 6, P = 0.01) for the TSHR w/w genotype and from occasion two to three for the d/d genotype (U = 0.0, n2
Discussion
The results from this study showed that the domestication related mutation in the chicken TSHR gene affects reproductive traits. TSHR expression in brain differed significantly between genotypes for both males and females. Males homozygous for the “domestic” allele (d/d) showed a reduced phenotypic response and lower levels of gene expressions in comparison to the w/w genotype when exposed to alterations in day length, and TSHR d/d females showed a faster onset of lay in comparison to w/w
Acknowledgments
This research was funded by The Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning, FORMAS (Formel Excel), The Swedish Research Council, VR and the European Research Council (ERC grant 322206 GENEWELL). Thanks to Lejla Bektic, Petros Batakis, Andrey Höglund and Ann-Charlotte Svensson, Linköping University, Sweden for technical assistance, and to Anna Johansson, Swedish University of Agricultural Sciences, Sweden, for providing us the Swedish landrace blood
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